Implement quiescence protocol

Quiescence is a new protocol feature that allows for channels to undergo
"fundamental" changes (i.e., protocol upgrade) while there are no
pending updates on either side. Its first use case will be to carry out
channel splices, to ensure new HTLC/fee updates are not made while a
splice is being negotiated.

Each side of the channel is allowed to send a `stfu` message if any of
their outbound updates are not pending for either side (i.e.,
irrevocably committed on both commitment transactions). Once both sides
exchange `stfu`, the channel becomes quiescent. A message timeout is
enforced during the quiescence handshake to ensure we can eventually
re-establish the channel and propose new HTLC/fee updates again.

Several new state flags have been added to `ChannelState::ChannelReady`
to track the progress of the quiescence handshake. Once the channel
becomes quiescent, we transition a new `ChannelState::ChannelQuiescent`
state. Since quiescence is not a persistent protocol (it implicitly
terminates upon peer disconnection), and updates cannot be made, we
chose to introduce a new state completely once quiescent, as several of
the `ChannelState::ChannelReady` flags (e.g., `AWAITING_REMOTE_REVOKE`,
`MONITOR_UPDATE_IN_PROGRESS`, etc.) are not relevant.

Author: wpaulino

lightning-types/src/features.rs

@@ -72,6 +72,8 @@
 //!     (see the [`Trampoline` feature proposal](https://github.com/lightning/bolts/pull/836) for more information).
 //! - `DnsResolver` - supports resolving DNS names to TXT DNSSEC proofs for BIP 353 payments
 //!     (see [bLIP 32](https://github.com/lightning/blips/blob/master/blip-0032.md) for more information).
+//! - `Quiescence` - protocol to quiesce a channel by indicating that "SomeThing Fundamental is Underway"
+//!     (see [BOLT-2](https://github.com/lightning/bolts/blob/master/02-peer-protocol.md#channel-quiescence) for more information).
 //!
 //! LDK knows about the following features, but does not support them:
 //! - `AnchorsNonzeroFeeHtlcTx` - the initial version of anchor outputs, which was later found to be
@@ -150,7 +152,7 @@ mod sealed {
 			// Byte 3
 			RouteBlinding | ShutdownAnySegwit | DualFund | Taproot,
 			// Byte 4
-			OnionMessages,
+			Quiescence | OnionMessages,
 			// Byte 5
 			ChannelType | SCIDPrivacy,
 			// Byte 6
@@ -171,7 +173,7 @@ mod sealed {
 			// Byte 3
 			RouteBlinding | ShutdownAnySegwit | DualFund | Taproot,
 			// Byte 4
-			OnionMessages,
+			Quiescence | OnionMessages,
 			// Byte 5
 			ChannelType | SCIDPrivacy,
 			// Byte 6
@@ -534,6 +536,16 @@ mod sealed {
 		supports_taproot,
 		requires_taproot
 	);
+	define_feature!(
+		35,
+		Quiescence,
+		[InitContext, NodeContext],
+		"Feature flags for `option_quiesce`.",
+		set_quiescence_optional,
+		set_quiescence_required,
+		supports_quiescence,
+		requires_quiescence
+	);
 	define_feature!(
 		39,
 		OnionMessages,
@@ -1175,6 +1187,7 @@ mod tests {
 		init_features.set_channel_type_optional();
 		init_features.set_scid_privacy_optional();
 		init_features.set_zero_conf_optional();
+		init_features.set_quiescence_optional();
 
 		assert!(init_features.initial_routing_sync());
 		assert!(!init_features.supports_upfront_shutdown_script());
@@ -1195,7 +1208,7 @@ mod tests {
 			assert_eq!(node_features.flags[1], 0b01010001);
 			assert_eq!(node_features.flags[2], 0b10001010);
 			assert_eq!(node_features.flags[3], 0b00001010);
-			assert_eq!(node_features.flags[4], 0b10000000);
+			assert_eq!(node_features.flags[4], 0b10001000);
 			assert_eq!(node_features.flags[5], 0b10100000);
 			assert_eq!(node_features.flags[6], 0b00001000);
 		}